The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications and broadcast services. Antenna systems for such spacecraft may include array-fed reflectors, for generating shaped beams corresponding to specific antenna pattern coverage requirements.
A feed array configured for the transmission of RF energy therethrough may be communicatively coupled with an antenna reflector and may include an array of multiple feed elements configured as horns. Center-to-center spacing dc-c between adjacent horns in such a feed array is, desirably, made as small as possible in order to provide a maximal degree of pattern control for the shaped beam. For horns having a circular aperture, based on cutoff of the dominant circular waveguide mode, dc-c should be no less than approximately 1.2λ, where λ is the wavelength corresponding to the lowest frequency of the RF energy (the “characteristic wavelength”). Moreover, dc-c must exceed the horn aperture outer diameter, da, so as to ensure a positive “gap” between horns at the aperture plane. This gap may be for example, about 1/20th of the aperture diameter.
Aperture efficiency, which may be characterized by a metric referred to as peak directivity, is a critical performance metric for feed array elements. For example, the achievable edge of coverage (EOC) secondary pattern directivity for the shaped beam directly tracks the radiating element's peak directivity. A 0.1 dB decrease in primary pattern peak directivity may result in a 0.1 dB decrease in secondary pattern EOC directivity.
Other important performance metrics for the feed array elements include polarization purity (or, equivalently, suppression of cross polarization) and radiation efficiency, i.e., the fraction of available power that is actually radiated by the element. Radiation efficiency incorporates the effects of impedance mismatch (return loss) and dissipation loss.
For closely spaced arrays, mutual coupling between neighboring elements can perturb and degrade the radiating elements' performance as reflected in one or more of the above mentioned metrics.
Performance degradation due to mutual coupling must be accommodated in communication system link budgets or suppressed. Known suppression techniques entail the use of additional components arranged between the radiating elements. For example, U.S. Pat. No. 2,987,747 to Atchison discloses that adjacent radiating elements may be shorted together at a distance of one quarter wavelength from a common aperture plane, generating an RF choke that inhibits mutual coupling. U.S. Pat. No. 4,115,782 to Han discloses metal tabs or clips inserted near the apertures of radiating elements to reduce mutual coupling effects. U.S. Pat. No. 4,219,820 to Crail discloses a planar metallic shape etched on a dielectric substrate that is inserted into the aperture of circular horn elements to provide coupling compensation between circularly polarized horn antennas to reduce degradation of polarization purity. Techniques disclosed in the above mentioned references rely, undesirably, on additional components, the installation, calibration and test of which add appreciably to the cost of the array, and which represent additional failure mechanisms that detract from the array's reliability.
Thus, improved techniques for reducing mutual coupling between radiating elements are desirable.